This invention relates to a method of detecting the temperature of brake fluid which is effective in controlling the brake fluid pressure in an automotive brake using electromagnetic on-off valves or a spool/valve type electromagnetic proportional pressure control valve, and a method of controlling the brake fluid pressure which makes it possible to control the brake fluid pressure to an ideal state while avoiding bad influences due to change in the viscosity of the brake fluid with change in the temperature.
Automotive brake systems are being sophisticated year after year. Recent brake systems include not only ABS's (antilock brake systems) but TCS's (traction control systems) and ASC systems (active safety control systems for correcting oversteering or understeering while the vehicle is turning by individually controlling the wheel brakes).
Such a brake system is shown in FIG. 3. In this system, during normal braking, the fluid passage connecting a master cylinder 2 to a wheel cylinder 4 for producing braking force is open, so that brake fluid can freely flow therebetween.
When a brake pedal is in its ON position, if an electronic control unit (not shown) detects any lockup tendency of the vehicle wheel and produces a pressure reduction signal, the position of an electromagnetic changeover valve 5 changes over to disconnect the wheel cylinder 4 from the master cylinder 2, while an electromagnetic on-off valve 9 opens to discharge fluid pressure from the wheel cylinder 4 into a reservoir 3.
When the locking tendency of the wheel disappears as a result of the pressure reduction, the electronic control unit detects this fact and produces a pressure re-increase signal. In response to this signal, the electromagnetic on-off valves 8 and 9 are opened and closed, respectively, so that fluid pressure is supplied into the wheel cylinder 4 from a pump 6. The wheel cylinder pressure thus rises again. Alternatively, the electromagnetic on-off valves 8, 9 may be both closed to maintain the wheel cylinder pressure. During antilock control, the above operations are repeated until the vehicle comes to a stop or until the brake pedal 1 is released to prevent lockup of the vehicle wheel.
Traction control is similar to antilock control except that the brake pedal 1 is not trodded during traction control. If the electronic control unit detects slip of the vehicle wheel, the electromagnetic changeover valve 5 changes over and the electromagnetic valve 8 opens so that fluid pressure is supplied into the wheel cylinder 4 from an accumulator 7. The wheel cylinder 4 is thus braked in spite of the fact that the brake pedal is not trodden. Then, pressure reduction and pressure re-increase operations are repeated to prevent slip of the vehicle wheel.
In a different arrangement, an electromagnetic proportional pressure control valve 10 shown in FIG. 4 is used to introduce fluid pressure from the pump circuit into the wheel cylinder 4 and to discharge fluid pressure from the wheel cylinder 4 into the reservoir 3.
This electromagnetic proportional pressure control valve 10 comprises a housing 11, a spool 12 substantially liquid-tightly and slidably inserted in the housing, a reaction pin 13 inserted at one end of the spool 12, a spool-biasing spring 14, and an electromagnet 15 for biasing, i.e. pulling, the spool 12 in the direction opposite to the direction in which the spool is biased by the spring 14.
The housing 11 has a first port 16, a second port 17, a third port 18, a first fluid chamber 19 into which one end of the spool 12 protrudes, and a second fluid chamber 20 into which the other end of the spool 12 protrudes.
The spool 12 has a surface passage 21, and an internal passage 22 kept in communication with the second port 17. The internal passage 22 has one end open to the first fluid passage 19, and at this end, the reaction pin 13 is substantially liquid-tightly inserted in the passage 22. Thus, a difference equal to the sectional area of the reaction pin 13 is created between the areas for bearing fluid pressures that urges the spool 12 in opposite directions. The spool 12 is thus biased under a downward thrust which is equal to the above difference in area multiplied by the pressure at the second port 17.
Between the spool 12 and the first port 16, a first valve portion 23 is formed to open and shut off communication between the first and second ports 16, 17 according to the position of the spool. Between the spool 12 and the third port 18, a second valve portion 24 is formed to open and shut off communication between the second and third ports 17, 18 according to the spool position. The degree of opening of each of the first and second valves 23, 24 changes with the spool position.
With this electromagnetic proportional pressure control valve 10, during a non-control state in which no current is supplied to the electromagnet 15, the spool 12 is maintained in the illustrated position by the spring 14. In this state, the first valve portion 23 is open, so that fluid pressure from the first port 16 flows into the second port 17.
When the electromagnet 15 is energized, the spool 12 is pulled downward in the figure by the electromagnetic force until the upward force balances with the downward force.
The relation at the balancing point is given by the following formula (1). Until the first valve portion 23 closes, the pressure at the second port 17 and the spool moving distance increase as the exciting current I increases. When the current I further increases after the first valve portion 23 has been closed, the second valve portion 24 will open, thus communicating the second port 17 to the third port 18. The pressure at the second port 17 thus drops. EQU Fpr+Fsol=Fsp (1)
Fsp: force of the spring 14 PA1 Fsol: driving force by the electromagnet 15 PA1 Fpr: thrust resulting from fluid pressure
Fpr in the above equation is given by: EQU (P2-P3).multidot.S
wherein P2 is the pressure at the second port 17 (load pressure), P3 is the reservoir pressure, and S is the sectional area of the reaction pin 13. On the other hand, Fsol equals b.multidot.I2 (b is a constant). Thus, the following relations are met: EQU (P2-P3).multidot.S+b.multidot.I2=Fsp EQU .thrfore.P2=(Fsp-b.multidot.I2)/S+P3 (2)
Since Fsp, b, S and P3 are all constants, the pressure P2 is proportional to the current I. In the equation (I), (Fsp-Fsol) is the spool driving force by the driving means.
In the arrangement in which the electromagnetic on-off valves 8, 9 shown in FIG. 3 are used to introduce fluid pressure from the fluid pressure source (pump) into the wheel cylinder and discharge fluid pressure from the wheel cylinder into the reservoir, if the viscosity of brake fluid changes markedly, while the viscosity is extremely high, brake fluid flows at a slow rate, so that the amount of fluid that passes through the on-off valves decreases. This creates a difference between the pressure range when the fluid viscosity is low (shown by solid line FIG. 5) and the pressure range when it is high (chain line).
In the arrangement in which the spool-valve type electromagnetic proportional pressure control valve shown in FIG. 3 is used to control brake fluid pressure, if the fluid viscosity is extremely high, the actual pressure P rises or falls only slowly as shown in FIG. 6, so that it takes a long time for the actual pressure P to reach the target pressure P(n). This means delay in response.
An object of this invention is to provide a method of detecting a brake fluid temperature as a basic data for control without a temperature sensor and a method of controlling brake fluid pressure which can control fluid pressure based on the detected data so that sufficiently accurate control is possible even if the fluid temperature is low and thus its viscosity is extremely high.